Hello,

If you are looking for some kind of pagination, then this option is available in Lightbox gallery web part configuration menu - check Enable paging and set Page size.

On the other hand, if you want a user/editor to choose which images from media library he wants to display in the content of the web part, you will have to develop a custom form control because there is no built-in one which would enable you to choose multiple images at once and also modify code of the web part a little. More information about this can be found in Developer's Guide - Developing form controls, Modifying the code of standard web parts.

Best regards,
Filip Ligac